Objective 1a, for work in Guatemala (Central Endemic Zone). Onchocerciasis is still a public health problem for several countries around the world. In the Americas, efforts implemented and sustained by regional initiatives have achieved significant advances on reducing the transmission of the disease. In Guatemala, after 17 years of uninterrupted mass drug administration with ivermectin (MDAI), transmission has been declared eliminated in three of the four foci. Post-treatment evaluation of the fourth focus is planned to start late on 2013 and, if no evidence of transmission if found, the country will be able to request evaluation of the elimination of the disease on 2015. On this context, we propose to describe the humoral immune profile of subjects living in former endemic areas for onchocerciasis before and after cessation of MDAI by assessing onchocercal biomarkers. Levels of IgG4, IgG3 and IgE immunoglobulins directed against different Onchocerca volvulus antigens will be assessed by ELISA, as well as levels of Th type 1 and type 2 immune response related cytokines and chemokines (IL-4, IL-5, IFN-gamma, MDC/CCL22 and TARC/CCL17). The long term evolution of each biomarker will be used to assess the effectiveness of each of the evaluated immune response components as a surveillance tool through MDA and after disease transmission has been interrupted. Also, these immune system components will be used to establish immunological profiles of infected, previously infected and non-infected individuals living in the central endemic zone of Guatemala. These immunological profiles, re-evaluated for each group over time, will be used to develop a model to classify infection status, aiming to resolve the diagnosis of possible cryptic onchocerciasis infections, in order to improve disease surveillance during MDAI and after treatment interventions stop. An improved surveillance would contribute to avoid reemergence or reintroduction of the disease in the future. Study results and the humoral immune pattern strategy developed may be adjusted and applied to other endemic regions of the world where onchocerciasis is still endemic, even during or after stopping MDAI.